Digital images are typically defined by a two-dimensional array of picture elements (pixels). In a typical digital monochrome image, each pixel is assigned a one-dimensional gray-scale (lightness, brightness, intensity) value; in a typical digital color image, each pixel is assigned a three-dimensional color vector (e.g., in an RGB color space, each pixel is assigned three values, one for red, one for green, and one for blue). The values are typically integers, so shades and colors corresponding to non-integer values are represented by (clipped to) the nearest integer value. However, the color-resolution limits of human perception are such that assigning eight bit values per color per pixel can yield images that are difficult to distinguish from continuous tone. For this reason, 8-bit-per-color-per-pixel images are considered continuous-tone herein.
Printers employing discrete device state marking technologies, such as many inkjet, offset, dry electrophotograpy (e.g., “laser”), liquid electrophotography, often face a challenge in reproducing continuous-tone images. Many such printers have too few different color or shade of inks available to produce all shades or colors of a continuous-tone image, even when superimposing ink dots to yield combination colors. Some high-end ink-jet printers alleviate this problem by using more ink colors, e.g., 6-12 inks at a time instead of 3-4. This can be a costly solution and does not in and of itself achieve direct continuous-tone printing.
Accordingly, discrete-device-state printers take advantage of a tendency in human perception to spatially average high frequency visual stimuli, interspersing colors according to a half-tone technique to give the perception of intermediate colors that are not directly producible. Newspapers and other media have long interspersed black dots and white dots (absence of black ink on a white print medium) to give the impression of shades of gray. Likewise, discrete colors can be interspersed to give the perception of intermediate colors. Thus, half-toning (among other things) has helped discrete-device-state printers generate pleasing continuous-tone images using a limited color palette.
Herein, related art is described to facilitate understanding of the invention. Related art labeled “prior art” (if any) is admitted prior art; related art not labeled “prior art” is not admitted prior art.